The present invention is related to radiation shield coating in diagnostic x-ray applications. It finds particular application in coating the housing of x-ray tubes or the walls of an x-ray examination room to prevent the unwanted propagation of x-radiation therefrom.
X-ray tubes are used in radiographic or fluoroscopic imaging to produce x-rays that are directed towards an object being imaged. These x-ray tubes are usually enclosed in a housing that, among other things, prevent x-rays from propagating in undesirable directions. To this end, the inside of these housings are usually lined with lead sheets that have been cut from larger sheets of lead and formed to fit along the inside surfaces of the housing. Similarly, rooms in which x-ray examinations are conducted are also lined with lead sheets to prevent unwanted propagation of x-radiation outside of the room.
The use of lead sheets for gamma ray shielding is set forth in U.S. Pat. No. 5,334,847 to Kronberg ('847 patent). Many of the reasons set forth in the '847 for using lead sheets for gamma ray shielding are also applicable to x-ray shielding. Specifically, lead is often used as a shield because it is dense, easily worked, relatively inexpensive and is often smaller than a comparable radiation shield made of almost any other material, and therefore it takes up less space.
A drawback with using lead sheets for radiation shielding, however, is the time required to cut each sheet to an appropriate size and apply the cut sheet to the housing or wall. Another drawback arises in accurately cutting the lead sheet so that there are no voids at a seam or Junction of adjoining sheets when they are applied to the housing or wall. Still another drawback with lead sheets lies in applying a fiat sheet of lead to a curved surface as might occur in a housing for an x-ray tube. Yet another drawback, set forth in the '847 patent, is that lead is a toxic metal that is slowly attacked and corroded by air, water and soil acids. In spite of these drawbacks, however, lead sheets have been successfully and safely used to line the housings of x-ray tubes and the walls of x-ray examination rooms.
In an effort to overcome the above drawbacks other metals that provide radiation shielding have been proposed. One such metal is bismuth which appears next to lead on the periodic table. Some of properties of bismuth include: the density of bismuth is 9.75 grams/cubic centimeter (86% the density of lead); 99%+ purity bismuth is relatively inexpensive; bismuth is relatively brittle; bismuth is immune to corrosion under most environments; and bismuth forms salts that hydrolyze in water to become insoluble and therefore it is virtually non-toxic. Because of its density, bismuth is an effective radiation shield. Moreover, because of its availability, low cost and because it is virtual non-toxic, bismuth is a desirable alternative to lead as a radiation shield. However, because bismuth is relatively brittle, it is difficult to form small sheets of bismuth from larger sheets. Thus, its usefulness is limited in application where custom sheet sizing is required. Such applications include x-ray tube housings and the walls of x-ray examination rooms
One use of bismuth in a radiation shielding environment is set forth in the '847 patent wherein bismuth is used as an exterior coating for a container used to transporting or storing radioactive material. Specifically, molten bismuth is applied to the exterior of the container which is made from a material, such as depleted uranium. More specifically, the container is placed in a mold made from a high-melting metal to which bismuth does not adhere, such as stainless steel, and molten bismuth is poured into the mold. The container is positioned in the mold so that the molten bismuth poured into the mold covers the entire surface of the container. The molten bismuth "Hans" with the depleted uranium in a manner similar to the way tin or its alloys coat and adhere to copper or brass. As a result of "tinning", the molten bismuth spreads over the surface of the depleted uranium and adheres strongly to the depleted uranium.
A drawback of using molten bismuth is that a mold is needed to apply the molten bismuth to the housings of x-ray tubes or the walls of x-ray examination rooms. Another drawback is that molten bismuth is used whereas it is desirable to work with bismuth at room temperature.
The present invention contemplates an improved radiation shield coating and method for applying the same that overcomes the above problems and others.